The invention relates to devices for detecting and warning a driver of an improper amount of air pressure in a pneumatic tire on the vehicle he is driving, and more particularly to devices for warning the vehicle driver when a condition exists which is indicative of either an improperly inflated tire or a shock absorber needing adjustment or replacing.
The added expense of an under inflated tire or a maladjusted shock absorber is well known. Under inflated tires reduce fuel economy by providing unnecessary rolling resistance, and also exhibit shorter useful lives due to excessive flexing of the tire side walls. Maladjusted shock absorbers not only tend to further decrease the useful life of the tires with which they are connected, but they also tend to shorten the useful life of the entire vehicle suspension system, by failing to dampen the amount of stress experienced by other components of this system. Although these costs are well known, the typical motorist does not often take the additional time required to manually measure the pressures of his various tires. Likewise, the typical motorist does not have ready access to the conventional tools or methods used to detect early failure of a shock absorber.
Many attempts to overcome these problems have been made. Several of these consist of placing a visual tire pressure indicator on the valve stem of a tire. Unfortunately, these require the driver to visually inspect each tire, which is a task that cannot be done while the vehicle is traveling. To overcome this problem, others have incorporated a remote signaling device for use in conjunction with other types of stem-located indicators for signaling information regarding a tire's pressure to the driver. However, these tend to produce a signal which is subject to false alarms introduced by bounce or the centrifugal force of the tire, and tend to unbalance the tire to which they are attached.
Several additional remote systems for warning the driver of an under inflated tire while he is driving have been attempted. These have included the incorporation of tire pressure monitors located inside the tires being monitored, which are associated with communication devices for transmitting a warning to the driver when the pressure in one of the tires falls below a predetermined value. An example of these devices is disclosed in U.S. Pat. No. 4,240,061 (Cohen), in which a sensing device and transmitter are located inside each pneumatic tire for rotating with the tire. The same signal reliability problems due to system stresses caused by centrifugal force and bounce noted earlier limit these type systems.
In U.S. Pat. No. 4.978,941 (Brown) is disclosed a remote system in which the suspension system and centrifugal stresses noted above are reduced, via locating the pressure sensor and transmitter devices in close relation to the center of each tire. Although the problem stresses are addressed in this teaching, the expense of the special tire mounts disclosed therein would be prohibitive for the typical motorist.
Another type system disclosed in the prior art is that of monitoring distances between fixed portions of the vehicle and the road surface, in relation to a predetermined value, such as the distance between said fixed portions when the tire adjacent said fixed portions was last inflated. Clothier, in his U.S. Pat. No. 4,075,602, describes in one of these type systems firmly flexible wires fixedly mounted adjacent each tire. Each wire is fixed so that it will not touch the road surface, unless the pressure of the tire with which it is associated falls below a predetermined amount, as indicated by said tire going "flat", and thereby allowing said wire to touch the road. The wires are electrically connected to a monitoring system for precipitating a warning signal to the driver when one of said wires touches the road surface. Although these type systems appear relatively simple, they are prone to false alarms caused by the many variations in road surface, such as bumps and ruts, over which most vehicles tend to drive and tend to require more additional maintenance effort, such as readjusting said wires after each reinflation of the tires, than most motorists are willing to add to their schedule.
Another type device is taught by Jones in U.S. Pat. No. 4,574,267, in which is disclosed the use of an accelerometer in conjunction with a remote signaling device and computer for measuring the changes in acceleration of a tire. The system monitors the acceleration rate and calculates a corresponding resonant frequency curve for each tire. This data is then compared to previous data and to similar data of another tire of the vehicle to detect an indication that one of the tires is under inflated. Although Jones' disclosure addresses many of the problems associated with prior systems, his disclosure is limited to the calculation and analysis of resonant frequencies associated with tire acceleration, which limits said system to the use of at least one micro-processor which tends to make the system too expensive for use by the average motorist.
While the above mentioned devices may be suitable for the particular purpose to which they address, they would not be as suitable for the purposes of the present invention. As well as having the problems of limited reliability and expense, none of the noted devices discloses a system for warning the driver of a condition in which a tire is improperly inflated or in which a shock absorber needs adjusting or replacing via monitoring the vertical distance through which the movable portion of a shock absorber travels, or the oscillation frequency said movable portion travels through said distance, during vehicular movement.
In view of the foregoing disadvantages inherent in the known types of tire pressure indicators now present in the prior art, the present invention provides an improved apparatus for warning a vehicle driver of a condition in which a vehicle tire is improperly inflated, or in which a shock absorber needs adjusting or replacing. As such, the general purpose of the present invention, which is described in greater detail below, is to provide a new and improved tire pressure- and shock absorber- monitor which has all the advantages of the prior art and none of the disadvantages.